A mixture formed of liquid photosensitive monomers has heretofore been used as an optical waveguide core layer-forming material intended for a mixed flexible printed circuit board for optical/electrical transmission, and in formation of the pattern of a core layer using the mixture, a desired core pattern has been produced by performing ultraviolet (UV) irradiation through a photomask. In general, a blending design using a resin having high aromaticity has heretofore been performed because such core layer-forming material is required to have a refractive index higher than that of a cladding layer-forming material. General approaches to the blending design for the alleviation of an optical loss at a wavelength of 850 nm in the optical waveguide core layer-forming material are mainly, for example, as described below (Patent Literatures 1 and 2). The transparency of the forming material is improved on the basis of the purity of a resin component of the forming material, thereby hindering the absorption peak thereof from shifting from a short-wavelength region to a wavelength of 850 nm, and particularly in the case of a photocurable resin material, the usage amount of a photopolymerization initiator is reduced.